PCI: Update code comment on PCI_EXP_LNKCAP_SLS for PCIe r3.0

Niklas notes that the code comment on the PCI_EXP_LNKCAP_SLS macro is
outdated as it reflects the meaning of the field prior to PCIe r3.0.
Update it to avoid confusion.

Closes: https://lore.kernel.org/r/70829798889c6d779ca0f6cd3260a765780d1369.camel@kernel.org
Link: https://lore.kernel.org/r/6152bd17cbe0876365d5f4624fc317529f4bbc85.1734376438.git.lukas@wunner.de
Reported-by: Niklas Schnelle <niks@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>

include/uapi/linux/pci_regs.h

@@ -533,7 +533,7 @@
 #define  PCI_EXP_DEVSTA_TRPND	0x0020	/* Transactions Pending */
 #define PCI_CAP_EXP_RC_ENDPOINT_SIZEOF_V1	12	/* v1 endpoints without link end here */
 #define PCI_EXP_LNKCAP		0x0c	/* Link Capabilities */
-#define  PCI_EXP_LNKCAP_SLS	0x0000000f /* Supported Link Speeds */
+#define  PCI_EXP_LNKCAP_SLS	0x0000000f /* Max Link Speed (prior to PCIe r3.0: Supported Link Speeds) */
 #define  PCI_EXP_LNKCAP_SLS_2_5GB 0x00000001 /* LNKCAP2 SLS Vector bit 0 */
 #define  PCI_EXP_LNKCAP_SLS_5_0GB 0x00000002 /* LNKCAP2 SLS Vector bit 1 */
 #define  PCI_EXP_LNKCAP_SLS_8_0GB 0x00000003 /* LNKCAP2 SLS Vector bit 2 */